Alouette-ISIS radio wave studies of the cleft, the auroral zone, and the main trough and of their associated irregularities


  • D. B. Mularew


The principal high-latitude ionospheric features such as the cleft, the nighttime auroral zone, the main trough and their associated irregularities, as observed with the Alouette/ISIS topside sounders, are reviewed. These features can be clearly distinguished in Alouette 1 electron density contour plots as a function of latitude and height below 1000 km. Alouette 2 data recorded near apogee (3000 km) show the location of the light-ion (H+ and He+) trough as a function of local time and show it to be more closely related to the plasmapause than is the main trough. An attempt is made to clarify the location and terminology of troughlike structures. Regions of small-scale irregularities (tens of meters) are associated with the high-latitude ionospheric features. These irregularities are responsible for the presence of aspect-sensitive spread F on topside ionograms. The most intense spread is found in the cleft region. Spread F regions usually have a sharp equatorward boundary. Some recent unpublished work by the author indicates that the spread F boundary for the nighttime auroral zone tends to be located several degrees equatorward of the keV electron precipitation zone and is probably generated indirectly by this precipitation and convected equatorward. In the main trough region spread is sometimes present and other times absent. Some of the individual small-scale irregularities responsible for spread F can extend along magnetic-field lines for hundreds of kilometers.